One of the major advantages of powder-metallurgical manufacture of components is that it becomes possible, by compacting and sintering, to produce blanks of final or very close to final shape. There are however instances where subsequent machining is required. For example, this may be necessary because of high tolerance demands or because the final component has such a shape that it cannot be pressed directly but requires machining after sintering. More specifically, geometries such as holes transverse to the compacting direction, undercuts and threads, call for subsequent machining.
By continuously developing new sintered steels of higher strength and thus also higher hardness, machining has become one of the major problems in powder-metallurgical manufacture of components. It is often a limiting factor when assessing whether powder-metallurgical manufacture is the most cost-effective method for manufacturing a component. Hence, there is a great need for new and more effective additives to improve the machinability of sintered steels. It is then important that this additive does not appreciably affect the mechanical properties, such as tensile strength and elongation, of the sintered material.
Today, there are a number of known substances which are added to iron-based powder mixtures to facilitate the machining of components after sintering. The most common powder additive is MnS, which is mentioned e.g. in EP 0 183 666, describing how the machinability of a sintered steel is improved by the admixture of such powder. Materials which are difficult to machine, in this context materials having a hardness above about 180 HV, cannot however be machined properly by adding MnS. Moreover, depending of added amount and base-material, additions of MnS may reduce the mechanical strength of the material after sintering.
WO 91/14526 describes how small amounts of Te and/or Se together with MnS are used to improve the machinability about twice in powder-metallurgical materials that are difficult to machine. The addition of Te and/or Se is already conflicting with environmetal considerations, in that the hygienic limit values for these additives are very low and there is a tendency towards even more stringent environmental regulations.
U.S. Pat. No. 4,927,461 describes the addition of hexagonal BN (boron nitride) to iron-based powder mixtures to improve machinability of the metal part after sintering. In the patent it is stated that by using agglomerates of very fine BN powder, it is possible to achieve a similar improvement of the machinability as by the addition of MnS. However, the sintered strength is affected to a lesser extent if a correct amount of BN powder is added, than if MnS is added.
Also the U.S. Pat. No. 5,631,431 relates to an additive for improving the machinability. According to this patent the additive contains calcium fluoride particles which are included in an amount of 0.1-0.6% by weight in the powder composition. In practice calcium fluoride has turned out to be an excellent machinability improving agent. However due to the continuous development of PM materials there is a need to improve the performance of the additives as well.
Thus an object of the present invention is to provide a new additive for a powder metal composition for further improvement of machinability. Another object of the invention is to provide a new additive which has no or essentially no influence of the mechanical properties. Additionally the new additive should be environmentally acceptable.